Maybe you can find a "cloud provider" with 970, 980, 980ti, 1080, 1080ti etc; probably not, because they do not focus on hash cracking but on other types of computations (which include/focus on floating point instructions etc).

They might not be (too) slow (e.g. compared to even older GPUs or CPUs etc), but as said they could be much slower than a very cheap consumer GPU that you get for around 150$.

Of course more cores means more performance (in general), but here we are talking about some very different architectures and instruction sets. That is what makes the real difference for hash cracking.

Maxwell is currently much better when it comes to hash cracking than Kepler (Teslas etc)